When I recieve an invitation (via email) it pops up in my calendar. I then click it to say accept. When I do my calendar shows that I have accepted, but the person who sent me the invitation does not see my status.

Both sender and recipient are using OSX Lion Server as Mail and Calendar servers and the standard calendar/mail app. With static IP-Adresses. Does iCal need a special port to be forwarded? Or do I need to set some flag in postfix's configuration so that iCal mails are relayed?

The servers relay outgoing mail through a separate host which requires authentication. Could it be that iCal does not use the authentication when it tries to relay the message?

Your Problem is not with iCal directly but with your Mail-Server, which is not properly configured. iCal is sending the invitation/confirmation via SMTP, but the Mail-Server of your Host/Internet-Provider is rejecting the mail due to an authentication error - hence the "Relay access denied". For further Information you can lookup the 554-Error in the official RFC-Documentation regarding SMTP (tools.ietf.org/html/rfc821), Chapter 4.2.1. 'REPLY CODES BY FUNCTION GROUPS' Do the Sender and Recipient use the same Mail/iCal-Server?
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J.C.Jul 3 '13 at 7:07

"Do the Sender and Recipient use the same Mail/iCal-Server?" No... similar setup two different machines... two different static ip addresses. Question: but doesn't the iCal-Answer-Mail get sent over the mac mail server too? It has the authentication information and has no problems with regular email.
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JosephJul 3 '13 at 8:48

My server supports "+" addressing. That isn't an issue. I checked the caldavd.plist but it has a completely different structure than the example in your first link. Is a separate user really necessary?
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JosephJul 3 '13 at 11:04